As the native population growth
rate has fallen, net foreign immigration has come to constitute a larger and
larger share of overall U.S. population growth. The size of this net inflow is
not known with precision, however, and this constitutes the largest unknown in
American population statistics. Despite the uncertainty, there has emerged a
consensus among demographers and public policy analysts on its probable range.

After
dropping to relatively low levels during the period 1931 to 1970, legal
immigration to the United States has increased significantly in recent decades,
primarily as a result of a change in immigration law in 1965. (see Table 1)
According to the U.S. Immigration and Naturalization Service (INS), 7.6 million
people attained legal permanent residence in the United States in the fiscal
year 1991 to 1998 period. That comes to an average of845,000 annually -- equaling the peak levels
of immigration early in the Twentieth Century.

Table 1. Legal immigration to the United States: 1901 to
1998

Years

Legal
Immigrants

Average per
Year

1901-10

8,795,386

879,539

1911-20

5,735,811

573,581

1921-30

4,107,209

410,721

1931-40

528,431

52,843

1941-50

1,035,039

103.504

1951-60

2,515,479

251,548

1961-70

3,321,677

332,168

1971-80

4,493,314

449,331

1981-90

7,338,062

733,806

1991-98

7,605,068

845,008

Source: 1998 INS Yearbook.

At the
end of the 1990s about 950,000 legal immigrants per year wer coming into the
U.S., according to the INS. If this figure is correct, immigration accounts for
more than one quarter of the country's total population increase. Consequently,
accurate estimates of the total U.S. population are highly dependent upon
accurate statistics on legal immigration.

How accurate is the immigration figure? Twenty years ago
population researcher Daniel R. Vining, Jr. tested the validity of consensus
immigration figures by estimating a plausible “lower bound” to net U.S.
immigration.(footnote 1)Vining focused
on one component of the net inflow of persons to the United States, namely,
their net flow by commercial airlines, which, assuming that net migration by
land, sea, and by clandestine and noncommercial air is positive, provides a
lower limit for total U.S. immigration.

A lower bound is only interesting, of course, if it proves
surprisingly high. Theofficial U.S.
government tally ofarriving and
departing air passengers certainly satisfies this criterion. In 2000, for
example, the excess ofarriving over
departing international air passengers was 4.718 million, according to the U.S.
DOT. (Table 2.) For the entire decade (1990 to 2000)it averaged 3.689 millionper annum. When Vining looked at this data in
the late 1970s, he found the excess to be about 1 million.

Table 2:International Air Passenger Arrivals to
and From the United States

Interestingly, while the number of international
passengers has risen by over 4-fold, the percentage difference between arriving
and departing international passengers, which Vining calls the “retention
rate,” has hardly changed at all: it was 7.8% in the 1970s, and was 7.7% in the
1990s. The constancy seems to imply that the impact of commercial air travel on
U.S. immigration has risen in lock step with the number of airline passengers
coming into the U.S.

The gap between international arrivals and departures (4.7
million in 2000) exceeds even the largest estimates made by demographers and
government population experts oftotal
net immigration into the United States. Part of the problem appears to be
attributable to a systematic undercount of departures on flights on flights
leaving the United States – but only a part. Vining found a systematic
undercount of passengers departing on charter flights in the government
statistics. The problem still exists: In contrast with most other countries,
the U.S. has no exit controls at airports. While the DOT and INS (and,
presumably, its successor agency) assures that the proper forms are filled out
on flights arriving in the U.S., both chartered and scheduled, because all
incoming passengers must pass through immigration and customs and because the
federal agencies want their own counts to tally with those of the air carriers,
they are less careful about recording outbound passengers. Outbound charter
flights in particular are cited as not complying with the paperwork, and thus
creating a large undercount of outbound international passengers.

Moreover, DOT rules regarding its basic international air
traffic report (the T-100 report) had, until recently, exempted air carriers
that operate aircraft with 60 seats or less. Therefore passenger volume in
particular markets – particularly the Caribbean and certain Canadian markets –
is understated due to the large amount of service provided by small aircraft.

Statistical coverage of international airline traffic was
the goal ofFederal legislation passed
in the wake of 9/11. The Enhanced Border Security and Visa Entry Reform
Act (HR 3525) that passed Congress last May requires U.S. citizens traveling
abroad to reveal their identities and travel plans. For the first time
government officials will monitor everyone who comes or goes from the United
States as part of an overall strategy to enhance national security. Expanded
monitoring is a first step toward a long-delayed automated entry/exit system
for tracking visitors at U.S. ports of entry.

Although electronic tracking of all
arriving and departing air passengers was to start on January 1, 2003, INS did
not publish the proposed new rule in the Federal
Register until February. The air passenger data requirement will not formally
take effect until the final rule is published, sometime after the 30-day
comment period. In addition, since there is normally a six-month lag before
airline passenger data are publicly released, comprehensive data on
international arrivals and departures will not be available until later this
year.

Vining's "solution" to flawed government
statistics was to use air passenger statistics independently collected by
individual airports as well as by the International Air TransportAssociation (IATA) on air passenger travel to
and from the U.S. over the North Atlantic – statistics that there is no reason
to believe undercount departures relative to arrivals. The non-government data
suggested a 50 to 60 percent smaller gap between arrivals and departures.
Vining reported that government data on scheduled internationalflights,
which in contrast to the charter flight data appear to be unbiased, were also
consistent with this estimate.

Unfortunately, recent figures on arrivals and departures on
scheduled international air flights are not readily available from DOT or the
IATA. However, if we applytheretention rate for scheduled flights reported
by Dr. Vining (3.8%) to current air passenger traffic volumes, we get an
estimate of2.4 million net arrivals in
2000 instead of the 4.7 million reported by the government. A more conservative
approach would simply cut Vining’s retention rate in half, giving us an estimated
million 1.2 million netarrivals in
2000.

Even this lower figure for airborne immigration is
substantially above recent INS estimates oftotal net immigration to the U.S., which, of course, includes net
migration by sea and land as well. The large disparity indicates either
increasingly large scale underreporting of departing air passengers (even on
scheduled flights) or a gross underestimate of U.S. immigration rates. Clearly
further research is needed to resolve the question.

1. Daniel R. Vining, Jr., “Net Migration by Commercial Air:
A Lower Bound on Total Net Migration to the United States,” Research in
Population Economics, Volume 4, pages 333-350, 1982.